This invention relates to signal translating circuits suitable for use in a recording amplifier provided for recording a video signal in a video tape recorder of the helically scanning type and the like.
There has been previously proposed such a circuit as shown in FIG. 1 for forming a recording amplifier used for recording a video signal in a video tape recorder of the helically scanning type. This circuit contains therein a signal translating circuit performing voltage to current conersion, a major part of which is formed into an integrated circuit 10 composed of transistors Q.sub.1 to Q.sub.9 and so on which are arranged on a common semiconductor substrate. In the integrated circuit 10, the transistors Q.sub.1 and Q.sub.2 constitute an input current mirror circuit portion which is supplied with a DC biasing current I.sub.B flowing therethrough by the transistor Q.sub.3 having its base supplied with a DC voltage V.sub.1. The transistors Q.sub.5 to Q.sub.8 constitute an output current mirror circuit portion and the transistor Q.sub.9 forms an output stage. The emitter of each of the transistors Q.sub.8 and Q.sub.9 is designed to be approximately fifty times as large in area as that of each of the transistors Q.sub.1 to Q.sub.7. An input terminal 11 is provided at the emitter of the transistor Q.sub.2 connected to act as a diode and an output terminal 12 is provided at the collector of the transistor Q.sub.9 . Further, a power supplying terminal 13 and a ground terminal 14 are also provided. At the outside of the integrated circuit 10, a source 20 of a video signal which is to be recorded is connected through a resistor R.sub.1 and a capacitor C.sub.1 to the input terminal 11, and a primary winding of a rotary transformer 30 is connected between the output terminal 12 and the power supplying terminal 13. A secondary winding of the rotary transformer 30 is coupled with a coil of a magnetic head 40.
In the recording amplifier thus constituted, the resistance value in the forward direction of the transistor Q.sub.2 connected to act as a diode and constituting the input current mirror circuit portion varies in response to a current flowing therethrough, and consequently an input signal current i.sub.IN is subjected to the second harmonic distortion thereof.
Generally, in the magnetic recording of a video signal, it is required to suppress a second harmonic distortion brought on a recording current to be less than minus forty decibels. For achieving such degree of suppression of the second harmonic distortion brought on the input signal current i.sub.IN with the recording amplifier shown in FIG. 1, it is necessary to cause the DC biasing current I.sub.B flowing through the input current mirror circuit portion to have a value more than twice as large as the peak to peak value of the input signal current i.sub.IN, as shown in FIG. 2. In the case where the DC biasing current I.sub.B has such large value, since the transistor Q.sub.4 is connected directly to the output current mirror circuit portion constituted by the transistors Q.sub.5 to Q.sub.8 to supply with the DC biasing current I.sub.B to the latter an output DC current I.sub.o flowing through the transistor Q.sub.9 forming the output stage is also caused to have a value more than twice as large as the peak to peak value of an output signal current. This results in a disadvantage that the power consumption is undesirably increased.
Further, in the recording amplifier shown in FIG. 1, since both the DC biasing current I.sub.B and the output DC current I.sub.o are determined inside the integrated circuit 10, the value of each of the DC biasing current I.sub.B and the output DC current I.sub.o varies in response to variations in temperature in the integrated circuit 10, and it is very hard to completely compensate the DC biasing current I.sub.B and the output DC current I.sub.o for variations in value by utilizing resistors provided in the integrated cicuit 10 or the base-emitter voltage of a transistor provided in the integrated circuit 10. This results in a defect that an output recording current varies in response to temperature variations in the integrated circuit 10. The fact that the output recording current is determined inside the integrated circuit 10 also causes a further disadvantage that respective output recording currents are lacking in uniformity in the case where plural recording amplifiers of the above mentioned type and each comprising the integrated circuit 10 are provided.